Larger minimoons arrive too, but less frequently. A minimoon about the size of a school bus probably comes and hangs out every half century or so; one the size of a football field may come about once every 100,000 years. "A hundred thousand years is about the time frame that human beings have been doing things like leaving their handprints on cave walls, so maybe in that time frame somebody once actually looked into the sky and saw a mini-moon moving across the sky," Robert Jedicke, who studies these minimoons, told National Geographic last year.

Their small size means that these minimoons -- despite their proximity, at least by space standards -- are difficult to spot. "So far, there's been just one confirmed sighting," writes Irene Klotz of Discovery News.

That may change soon. NASA was recently gifted two Hubble-quality spacetelescopes, and Jedicke and his colleagues are proposing we aim one of those badboys to search for these minimoons. His dream isn't just to watch these moons as they flit about the Earth, but to capture them.

"It's the Rosetta stone of the solar system. You bring back a chunk of material that's never been processed through the atmosphere, that's not been sitting on the ground. It's going to be a tremendous wealth of information about how the solar system formed -- even more so if you can bring back more than one and get different types of material," he told Klotz.